1. Field of the Invention
This invention relates to an optical measuring apparatus which measures physical quantity by employing an optical sensor that modulates intensity of light passing therethrough.
2. Description of Related Art
FIG. 1 is a block diagram to illustrate by way of example construction of a conventional optical measuring apparatus which was disclosed in the Japanese Utility Model Application Laid Open No. 61-125783(1986). In FIG. 1, reference numeral 1 designates an optical transmitter which is driven by a direct current (DC) and supplies light to an optical sensor 3. The optical sensor 3, whose optical characteristics were changed according to physical quantity of an object of measurement connected via an optical fiber 2 to the optical transmitter 1, and modulates intensity of the light supplied from the optical transmitter 1 according to Pockels effect of Faraday effect. An optical receiver 5 which is connected via an optical fiber 4 to the optical sensor 3 receives the light from the optical sensor 3 and converts this to quantity of electricity. A condenser 6 which is connected to the optical receiver 5 removes a DC component included in output of the optical receiver 5. A DC amplifier 7 which is connected to the optical receiver 5 amplifies the DC component alone which is included in the output of the optical receiver 5. An AC amplifier 8 which is connected via the condenser 6 to the optical receiver 5 amplifies output of the condenser 6. A divider 9 which is connected to both of the DC amplifier 7 and AC amplifier 8 divides output of the AC amplifier 8 by output of the DC amplifier 7. A comparator 10 which is connected to the DC amplifier 7 compares the output of the DC amplifier 7 with a specified value. The comparator 10 is connected to a display unit 11, which displays output of the comparator 10.
Now will be described below functional operation of the conventional optical measuring apparatus shown in FIG. 1. Light Pi which was driven with the DC current by the optical transmitter 1 and transmitted via the optical fiber 2 to the optical sensor 3, in which intensity of this light is modulated according to the physical quantity of the object of measurement. Intensity of light P.sub.0 modulated by the optical sensor 3 is obtained from the following formula. EQU P.sub.0 =k.multidot.Pi.multidot.(1+m) (1)
In this formula, k.multidot.Pi designates the average intensity of the light to be received by the optical receiver 5, so does m a modulation factor of the light whose intensity was modulated according to the physical quantity W of the object of measurement, then, there can be established the following formula: EQU m=W.multidot.sin (2.pi.ft) (2)
The light Po whose intensity was modulated by the optical sensor 3 is transmitted via the optical fiber 4 to the optical receiver 5 and is converted to an electric signal. For output of the optical receiver 5, the DC amplifier 7 amplifies a DC component alone and the AC amplifier 8 amplifies an AC component alone, respectively. Output of DC amplifier 7 Vdc and output of AC amplifier 8 Vac are to be given from the following formulas, where s.sub.1 is a coefficient given by multiplying a photoelectric conversion coefficient of the optical receiver 5 by an amplification rate of the DC amplifier 7, and s.sub.2 is a coefficient given by multiplying the photoelectric conversion coefficient of the optical receiver 5 by an amplification rate of the AC amplifier 8. EQU Vdc=s.sub.1 .multidot.k.multidot.Pi (3) EQU Vac=s.sub.2 .multidot.k.multidot.Pi.multidot.m (4)
When the output Vac of the AC amplifier 8 is divided by the output Vdc of the DC amplifier 7 by the divider 9, a signal V.sub.0 to be outputted to divider 9 is obtained from the following formula: ##EQU1##
The signal V.sub.0 depends only on the physical quantity of the object of measurement but not on the average intensity k.multidot.Pi of the light to be received by the optical receiver 5, accordingly, it is possible to measure the physical quantity with great accuracy.
The comparator 10 always monitors whether the level of the output of the DC amplifier 7 is below the specified value or not, and where not, the comparator 10 supplies its output to the display unit 11 to give an alarm. The value of the output of the DC amplifier 7 Vdc is in proportion to the average intensity of the light to be received by the optical receiver 5, so that the above monitoring enables the optical transmitter 1, optical fiber 2, 4, optical sensor 3, optical receiver 5 and DC amplifier 7 to self-check their malfunctions.
With the conventional optical measuring apparatus thus constructed, there has been a problem that it is impossible to self-check malfunctions in such other units as the condenser 6, AC amplifier 8 and divider 9.